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A metric tonne (1000 kg) should be called a megagram (1 Mg). (lemmy.ca)
submitted 3 years ago by to c/showerthoughts@lemmy.world
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[–] 1 point 3 years ago (1 child)

The mole is defined based on the gram and not the kilogram, even though the kilogram is the coherent unit of mass. I don't have an example, but it probably results in a bit of extra math somewhere. Again, who knows why. Apparently the mole has had conflicting definitions in the past, and one of them was based on the kilogram, so it seems like this would have been easy to do. Again, the gram is involved - maybe the two things are related?

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  • [–] 1 point 3 years ago (1 child)

    Apparently, the SI base units have been redefined, and the link between moles and kg was severed in 2019?

    I was vaguely aware of this shake-up after reading someplace that the kg had a new definition in terms of fundamental physical constants rather than the old one based on an official standard kg. This was basically a block of metal sitting in a lab someplace in France. But TIL other base unit definitions were also tweaked at that time.

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  • [–] 2 points 2 years ago*

    Their definitions are no longer related, but their sizes are still roughly the same relative to each other. I mean that the unit for amount of substance is based on 12 grams of C, instead of 12 kg of C, despite the kilogram being the unit for mass. Some fields used to use that unit and called it a kg-mole, but that notation would be pretty confusing and you would want to have a different name. https://en.m.wikipedia.org/wiki/Mole_(unit)

    I think the 2019 redefinition is really neat. They changed the system so that constants are defined instead of measured, in a way that makes estimates more precise. It's worth reading about if you're interested in the stuff.

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